This invention relates to hydropheumatic suspension systems for wheeled vehicles, especially four wheeled vehicles.
As is well known in the art, a hydropneumatic suspension system has a self-levelling or vehicle level adjusting function of maintaining the level of a vehicle body above the ground substantially constant independently of a load applied thereto, and this function makes it possible to give a soft springing of a vehicle, even in case of the vehicle which carries thereon a wide variety of loads, for thereby attaining a soft, cushioned ride.
However, in a vehicle equipped with such a hydropneumatic suspension system that is constructed to give a soft springing of the vehicle, a roll stiffness and a pitch stiffness of the vehicle becomes slightly smaller than required, which inevitably causes an appreciable side-roll of the vehicle body when the vehicle is under the conditions of making a turn and an appreciable fore-and-aft pitch of the vehicle body when the vehicle is under the accelerating or decelerating conditions, thus resulting in an uneasy feel of the passengers.
Also known in the art is a hydropneumatic suspension system of the type which is constructed to be able to lift a vehicle wheel above the ground, when required, through the operation of the suspension system of raising and lowering a vehicle body relative to the ground. This type of hydropneumatic suspension system is constructed in such a way as disclosed in, for example, U.S. Pat. No. 3,917,295.
In this prior art arrangement, a lift of a vehicle wheel is attained as follows: Firstly, the vehicle body is jacked up to its maximum level by supplying hydraulic fluid into the interiors of the suspensions units until the suspension units are put into their full-rebound conditions, i.e. the conditions in which the maximum extensible length of the suspension units is attained. Then, a suitable block is placed under the vehicle body so as to be located near the vehicle wheel to be lifted. After that, the hydraulic fluid in the suspension units is drained. Eventually, the vehicle wheel required is lifted by the action of an anti-roll bar disposed between the left and right suspension members and the effect of the tilt of the vehicle body. In this instance, it is an ordinary practice to determine the roll stiffness of the anti-roll bar according to the desired roll stiffness of the vehicle and the full-rebound stroke according to the desired riding comfort of the vehicle. In case that the full-rebound stroke is made longer than required for the riding comfort of the vehicle, the suspension units inevitably become of a considerable length and bulky, which is disadvantageous in view of a normal height of the vehicle body above the ground and arrangements of suspension elements.
For this reason, the method of lifting a vehicle wheel in the foregoing prior art arrangements cannot attain a sufficient lift of the vehicle wheel due to the lack of the full-rebound stroke of the suspension units and the stiffness of the anti-roll bar. This inconvenience is pronounced particularly in the case of a vehicle which is equipped with a suspension member of the type having a relative large unsprung weight, such as a rigid axle. In this case, though the lift of the vehicle wheel required can be attained to some extent under the conditions in which the tire of the vehicle wheel required to be lifted is not punctured, the lift of the vehicle wheel sufficient for tire replacement cannot be obtained in the case that a vehicle wheel with a punctured tire is to be lifted since the portion of the vehicle body where the vehicle wheel with a punctured tire is located remains lower than other portions of the vehicle body during jacking up of the vehicle body.